Fuel cells are suited for transportation vehicles because of their fast startup time, low sensitivity to orientation, and favorable power-to-weight ratio. Relatively low temperature operation at around 80° C. makes fast startup possible. Scarce, expensive platinum-based catalysts are needed for the oxygen reduction reaction (ORR) at the cathode of these fuel cells.
A need exists for less expensive catalysts that exhibit a catalytic activity similar to that for platinum-based metal catalysts.
Metal-nitrogen-carbon (M-N—C) type catalysts having non-precious metals have been studied for almost 50 years since the discovery of their ORR activity in macrocycles bound with non-precious metals. These M-N—C type catalysts are currently considered to be promising alternatives to platinum-based catalysts in fuel cells. M-N—C type catalysts that are iron-based, for example, are currently being investigated as possible alternatives to platinum-based cathode catalysts. For example, Lefèvre et al. in “Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells, Science, April 2009, vol. 324, pp. 71-74, incorporated by reference, reported the preparation of various iron-based M-N—C type catalysts for ORR. Lefèvre et al. identified the following factors for producing active Fe-based catalysts for ORR: (1) disordered carbon content in the catalyst precursor, (2) iron, (3) surface nitrogen, and (4) micropores in the catalyst. Lefèvre et al., noted targets set by the U.S. Department of Energy of 130 A/cm3 by 2010 and 300 A/cm3 by 2015 for volumetric activity as measured in a fuel cell at 0.8 V iR-free cell voltage (i.e. after correction for ohmic loss R) at 80° C., and at O2 and H2 absolute pressures of 1 bar and 100% relative humidity. According to Lefèvre et al., volumetric activity is a meaningful measure of activity because the product of volumetric activity with electrode thickness predicts the kinetic current density (in A/cm2) of the cathode. FIG. 1, taken from Lefèvre et al. is a plot of iR-free cell voltage vs. volumetric current density, including volumetric current density of their best (solid circle, 99 A/cm3) non-precious metal catalyst (NPMC). The original polarization curves of Lefèvre et al. were obtained from H2—O2 fuel cell tests at 80° C. and 100% relative humidity (smaller open circles represent PO2=PH2=1.5 bar). The smaller open diamonds are for data obtained for a catalyst reported by Wood et al., “Non-precious metal oxygen reduction catalyst for PEM fuel cells based on nitroaniline precursor,” J. Power Sources, 2008, vol. 178, pp. 510-516, incorporate by reference. FIG. 1 also shows corrected polarization curves (larger circles and larger diamonds) that are based on the DOE fuel cell test reference conditions (vide supra).